Cartan geometry of spacetimes with a nonconstant cosmological function Lambda
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We present the geometry of spacetimes that are tangentially approximated by de Sitter spaces whose cosmological constants vary over spacetime. Cartan geometry provides one with the tools to describe manifolds that reduce to a homogeneous Klein space at the infinitesimal level. We consider a Cartan geometry in which the underlying Klein space is at each point a de Sitter space, for which the combined set of pseudoradii forms a nonconstant function on spacetime. We show that the torsion of such a geometry receives a contribution that is not present for a cosmological constant. The structure group of the obtained de Sitter-Cartan geometry is by construction the Lorentz group SO(1, 3). Invoking the theory of nonlinear realizations, we extend the class of symmetries to the enclosing de Sitter group SO(1, 4), and compute the corresponding spin connection, vierbein, curvature, and torsion.